The invention is concerned with a transmitter for wireless remote control systems. The receiver in such systems normally includes a transducer for converting acoustic control signals into corresponding electrical signals, decoding circuits for determining which function has been selected, and utilization circuits for performing the selected functions. Of principal interest is the transmitter producing the control signals.
It is well known that by appropriately striking rods of appropriate mass and length one can produce acoustical signals of sufficient frequency stability and duration for use as actuating or control signals in remote control systems. There have been many such devices on the market. This invention concerns an electronic transmitter which does not use the mechanical striking of a rod to produce the acoustic control signal.
There are several forms of electronic transmitters available. They generally include tank circuits in which various capacitor or inductor combinations are connected to produce the desired control frequencies. Usually a double pole switch connects the particular reactive element combination to produce the resonant frequency and applies power to turn on the oscillator in that sequence. Various forms of output circuits such as electrostatic microphones are utilized. An electrostatic microphone requires additional power since its operation is dependent upon a sizable voltage being applied and hence provides a drain on the battery. An important consideration is that the frequency be independent of battery voltage.
The invention concerns an electronic transmitter for a remote control system which incorporates simple, economical single contact switches and a purely resistive network for selecting control frequencies. The invention includes a cascade arrangement of transistors in the form of an oscillator having positive and negative feedback loops. The frequency selective elements in the negative feedback loop cause a minimum amplitude zero phase shift condition in the network and thus establish the frequency of oscillation for the oscillator. The oscillator arrangement produces a control signal having a frequency and amplitude which are independent of battery voltage variations.
Moreover the absolute values of the resistive components are not significant. Rather it is the ratio of these resistive components which establishes oscillator frequency. Thus the invention is uniquely suited for production by integrated circuit techniques such as a thick film circuit.